- No concurrent cytochrome P450 enzyme-inducing antiepileptic drugs (e.g., phenytoin, carbamazepine, or phenobarbital), St. John's wort, ketoconazole, dexamethasone, dysrhythmic drugs (e.g., terfenadine, quinidine, procainamide, sotalol, probucol, bepridil, indapamide, or flecainide), haloperidol, risperidone, rifampin, grapefruit, or grapefruit juice